The present invention relates to preparation of extremely pure semiconductors.
The semiconductor HgCdTe has been a subject of development interest in the semiconductor art. The primary use of this material is for infrared imaging devices. In such applications, photosensing is accomplished by detecting carrier pairs generated in this very narrow bandgap material by direct absorption of infrared photons. For example, this may be done by sensing the amount of charge collected in a depletion well. However, the background carrier concentration is also available to fill these wells, so it is desirable to reduce this background carrier concentration as low as possible, to minimize dark current and improve the signal-to-noise ratio. This in turn requires keeping the residual dopant concentration as low as possible, preferably low enough to achieve a net carrier concentration below 1.times.10.sup.15 ions per cubic centimeter. This is not easy.
It is desirable to remove unwanted impurities, particularly group IB impurities, namely Cu, Ag, Au, which can diffuse into HgCdTe active device layers during conventional processing techniques. The terms HgCdTe herein represents Hg.sub.1-x Cd.sub.x Te where x ranges between 0 and 1. In the present invention, removal of these impurities is accomplished by solid state electromigration, whereby p-type (or rapidly diffusing n-type) impurities move as positively charged interstitials through the HgCdTe crystal lattice in the presence of an electric current passing through the crystal lattice. The presence of an electric current (not merely the overall field, but also the local perturbations due to an electron passing within a few lattice spacings of a mobile impurity) increases the probability of impurities to diffuse as positively charged interstitials toward a negative electrode maintained at the slice surface, thereby speeding the removal of the impurities from all port ions of the HdCdTe slice. (The transport mechanism is generally analog to ionic conduction in a solid electrolyte.) Diffusion of the impurities in the absence of an electric field would be random, and the probability of the impurities diffusing out of the HgCdTe slice would be low. It is desirable to lower the residual carrier concentration to less than 1.times.10.sup.15 /cm.sup.-3 in order to maximize the efficiency of HgCdTe infrared imaging devices, and removal of mobile positively charged impurities assists in this.
Existing techniques for reducing the residual acceptor concentration are undesirable since they cause decomposition of the HgCdTe material or longitudinal composition gradients within the HdCdTe ingots. There are no previously demonstrated non-destructive techniques other than the one described herein for reducing the residual acceptor concentration in HgCdTe material.
It is an object of this invention to provide a non-destructive method for purifying HgCdTe semiconductor slices in order to achieve residual acceptor concentrations less than 1.times.10.sup.15 /cm.sup.-3.
It is a further object of this invention to provide a method for the removal of Group IB impurities from HgCdTe semiconductor material without etching.
According to the present invention there is provided:
A method for purifying semiconductors, comprising the steps of:
making contact to a portion of a semiconductor to be purified;
immersing said semiconductor portion in a conductive liquid; and
passing current between said semiconductor portion and said conductive liquid;
whereby impurities electromigrate from said semiconductor into said liquid.